Copolymers of ethylene and chlorotrifluoroethylene are known for their chemical and mechanical resistance, in particular for their elevated resistance towards oxidising agents and highly basic solutions. These properties make the use of ethylene/chlorotrifluoroethylene polymers highly advantageous in membrane applications, in particular in the field of wastewater treatment where the cleaning of the membrane requires the use of highly oxidising and aggressive agents.
Membranes based on ethylene/chlorotrifluoroethylene polymers are known from the prior art. U.S. Pat. No. 4,623,670 (ASAHI CHEMICAL IND [JP]) Nov. 18, 1986 and U.S. Pat. No. 4,702,836 (ASAHI CHEMICAL IND [JP]) Oct. 27, 1987 disclose a process for preparing membranes made of a fluoropolymer, including copolymers of ethylene with chlorotrifluroethylene, said process comprising mixing 10-60% by volume of the polymer, 7-42% by volume of an inert compound, such as silica, and 30-75% by volume of chlorotrifluoroethylene oligomers as plasticizer; extruding the plasticized compound into pellets; subsequently melt-molding said pellets into articles, such as hollow fibers; removing by a first extraction the plasticizer from the molded article; and then removing by a second extraction, with a strong acid or base, the inert compound. This process has a number of drawbacks: it requires the working of the polymer at temperatures of 230-250° C. both during the plasticization and the molding step; the use of high amounts of the chlorotrifluroethylene oligomer plasticizer which has to be removed with chlorinated solvents, like 1,1,1-trichloroethane; the addition of an inert compound, to give sufficient mechanical properties to the polymer during the molding step, and which also has to be removed by extraction. The process is thus complicated and expensive.
EP 1236503 A (AUSIMONT SPA [IT]) Sep. 4, 2002 discloses a process for the preparation of porous membranes wherein ethylene/chlorotrifluoroethylene polymers, namely terpolymers comprising from 10 to 70% by moles of ethylene, from 30 to 90% by moles of chlorotrifluoroethylene, from 0.1 to 30% by moles, with respect to the total amount of ethylene and chlorotrifluoroethylene, of a hydrogenated monomer of formula: CH2═CH—(CH2)nR1 wherein R1═—OR2, or —(O)tCO(O)pR2 wherein t and p are integers equal to 0 or 1 and R2 is a linear or branched C1-C20 alkyl, or a cycloalkyl, optionally containing heteroatoms and/or chlorine atoms, are melt blended with a hydrogenated plasticizer, selected from the group of citrates, phthalates, trimellitates and the like, molded into an article by injection or compression molding at temperatures of about 200° C. and subsequently dipped into a solvent, such as a C1-C6 aliphatic alcohol, to extract the plasticizer.
WO 2005/037917 A (ZENON ENVIRONMENTAL INC) Apr. 28, 2005 discloses a temperature induced phase inversion process for membrane manufacture in which a polymer, inter alia an ethylene/chlorotrifluoroethylene polymer, is melt blended with a high boiling compound acting as a plasticizer, such as tri-isononyl trimellitate; shaped into an article either by molding or melt-spinning, optionally in the presence of a support; cooled to induce membrane formation and then subjected to an extraction treatment to remove the high boiling plasticizer.
WO 03/068374 A (US FILTER WASTEWATER GROUP INC) Aug. 21, 2003 discloses a temperature induced phase separation process for the production of ethylene/chlorotrifluoroethylene membranes wherein a blend of the polymer and a solvent, such as glycerine triacetate or citric acid acetyl ester, is prepared at a temperature in excess of 200° C.; the blend is formed into the appropriate shape and cooled until the polymer solidifies, then the solvent is extracted from the solidified membrane.
WO 2005/002712 (FILTER WASTEWATER GROUP INC) Jan. 13, 2005 discloses polymeric ultrafiltration or microfiltration membranes made from a variety of polymers, including ethylene-chlorotrifluoroethylene polymers and possibly modified by addition of polyvinylmethylether. Among others, manufacture of membranes made from HALAR® 901 by ‘TIPS’ technique, i.e. temperature induced phase separation process, is disclosed.
EP 1743921 A (SOLVAY SOLEXIS S.P.A.) Jan. 17, 2007 discloses thermoplastic compositions made of a halopolymer such as notably an ethylene/chlorotrifluoroethylene copolymer in admixture with other ingredients (a perfluoropolymer and a perfluoropolyether), which can be useful for the manufacture of several shaped articles, including, notably, membranes.
Thus, a need still exists in the art for a process for the production of ethylene/chlorotrifluoroethylene polymer membranes that does not require the working of the polymer at high temperatures. Also the need still exists for a process that does not require the use of high quantities of plasticizers and of lengthy extraction processes for their removal.
It has now been found that by using ethylene/chlorotrifluoroethylene polymers having a melting temperature not exceeding 200° C. it is possible to obtain membranes via a diffusion induced precipitation process.
The diffusion induced precipitation process, also defined as “diffusion induced phase-separation process”, is a well known process for the preparation of polymeric membranes whereby a polymer material is dissolved in an appropriate solvent to form a solution, the solution is cast in a membrane-forming layer which is then immersed in a non-solvent. The contact of the membrane-forming layer with the non-solvent induces the formation of two phases: a polymer-rich solid phase that forms the membrane structure and a solvent-rich liquid phase that forms the liquid filled membrane pores.
Advantageously the diffusion induced precipitation process does not require working of the polymer at temperatures higher than 200° C., i.e. at temperatures close to the ones at which the polymer may start to degrade. The process does not require the use of plasticizers, which have to be extracted with a suitable solvent and then recovered at the end of the membrane production process. Solvents of common use can be employed to prepare the polymer solution and water can be used as the non-solvent to induce both the precipitation of the polymer from the solution and, at the same time, the removal of the solvent.
Porous membranes of vinylidene fluoride polymers are typically produced using a diffusion induced precipitation process. The possibility of processing ethylene/chlorotrifluoroethylene polymers with the same technology as vinylidene fluoride polymers is a further advantage provided by the present invention.